On Fri, Nov 27, 2020 at 10:26:47AM +0000, Marc Zyngier wrote:
> On 2020-11-24 15:50, Will Deacon wrote:
> > If a vCPU is caught running 32-bit code on a system with mismatched
> > support at EL0, then we should kill it.
> >
> > Acked-by: Marc Zyngier <maz@xxxxxxxxxx>
> > Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> > ---
> > arch/arm64/kvm/arm.c | 11 ++++++++++-
> > 1 file changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 5750ec34960e..d322ac0f4a8e 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -633,6 +633,15 @@ static void check_vcpu_requests(struct kvm_vcpu
> > *vcpu)
> > }
> > }
> >
> > +static bool vcpu_mode_is_bad_32bit(struct kvm_vcpu *vcpu)
> > +{
> > + if (likely(!vcpu_mode_is_32bit(vcpu)))
> > + return false;
> > +
> > + return !system_supports_32bit_el0() ||
> > + static_branch_unlikely(&arm64_mismatched_32bit_el0);
> > +}
> > +
> > /**
> > * kvm_arch_vcpu_ioctl_run - the main VCPU run function to execute
> > guest code
> > * @vcpu: The VCPU pointer
> > @@ -816,7 +825,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
> > * with the asymmetric AArch32 case), return to userspace with
> > * a fatal error.
> > */
> > - if (!system_supports_32bit_el0() && vcpu_mode_is_32bit(vcpu)) {
> > + if (vcpu_mode_is_bad_32bit(vcpu)) {
> > /*
> > * As we have caught the guest red-handed, decide that
> > * it isn't fit for purpose anymore by making the vcpu
>
> Given the new definition of system_supports_32bit_el0() in the previous
> patch,
> why do we need this patch at all?
I think the check is still needed, as this is an unusual case where we
want to reject the mismatched system. For example, imagine
'arm64_mismatched_32bit_el0' is true and we're on a mismatched system: in
this case system_supports_32bit_el0() will return 'true' because we
allow 32-bit applications to run, we support the 32-bit personality etc.
However, we still want to terminate 32-bit vCPUs if we spot them in this
situation, so we have to check for:
!system_supports_32bit_el0() ||
static_branch_unlikely(&arm64_mismatched_32bit_el0)
so that we only allow 32-bit vCPUs when all of the physical CPUs support
it at EL0.
I could make this clearer either by adding a comment, or avoiding
system_supports_32bit_el0() entirely here and just checking the
sanitised SYS_ID_AA64PFR0_EL1 register directly instead.
What do you prefer?
Will
